In optical communication systems, optical signal are transmitted along an optical communication path, such as an optical fiber. Early systems deployed a single transmitter at a nominal wavelength at one end of an optical fiber link and receiver at the other end to detect incoming signals. More recently wavelength division multiplexed (WDM) system have been developed in which multiple colors or wavelengths of light are combined onto a single fiber in order to increase bandwidth of information carrying capacity of an optical communication network.
In a WDM system, a plurality of optical transmitters feed corresponding signals to an optical multiplexer is often provided at one end of an optical fiber link and an optical demultiplexer is provided at the other end to separate the WDM signal into individual optical signals at corresponding wavelengths. Often, however, network configurations may require that certain wavelengths be “dropped” or selected from the WDM signal prior to reaching the demultiplexer at the termination point of the fiber link. In addition, optical signals at the drop wavelength or other wavelengths may be required to be added prior to the termination point. Accordingly, so-called add-drop multiplexer have been developed to add/drop optical signals at certain wavelengths, while permitting optical signals at other wavelengths to pass to the termination point.
A conventional add/drop multiplexer is described, for example, in U.S. Pat. No. 6,459,516, incorporated by reference herein. This add/drop multiplexer can flexibly accommodate a relatively large number of added and dropped optical signal or channels. The channels that are added and dropped are fixed, however, and the add/drop multiplexer is not remotely reconfigurable.
An alternative add/drop multiplexer is the Select Optical Add/Drop Multiplexer (“S-OADM”) commercially available from CIENA Corporation of Linthicum, Md. As shown in FIG. 1, the S-OADM receives incoming optical signals through an optical amplifier 110. The optical signals are next passed to a power splitter or coupler 120, which supplies a first portion of each incoming optical signal to a reconfigurable blocking filter (RBF) 130 and a second portion of each signal to a pre-booster amplifier 160 and to router 180. Router 180 separates the WDM signal into separate channel groups, one of which is passed through a segment of dispersion compensating fiber (DCF) 121, and then to optical amplifier USA 197. The channel group is next fed to a channel group demultiplexer including 1×8 splitter 119, which supplies the channel group on each of eight outputs. Splitter 119 is a conventional power splitter so that the signal strength of each output is attenuated is about ⅛ the power of the incoming signal. Channel filters (not shown) are coupled to each output of splitter 119 to select individual channels from each splitter output and supply the demultiplexed channels to corresponding receivers (not shown).
Added channels are supplied from transmitters (not shown) to an 8×1 combiner 117 through amplifier 115 and to router 195. At the output of router 195, the added channel group is passed through an optional segment of dispersion compensating fiber 190, and amplified by amplifier 170. The channel group is next combined with channels output from RBF 130 by coupler 140, and the resulting WDM signal is output through amplifier 150.
In operation, the RBF is configured to block the channel group selected by port 161 of router 180, while remaining channel groups pass through. Although non-selected wavelengths are also supplied to router 180, no demultiplexing elements or receivers are provided to sense the non-selected wavelengths. The added channels are typically at the same wavelength as the blocked channels to prevent interference between those signals passed through RBF 130 and those, which are added. Alternatively, the added channels may be different from any of the pass through channels.
Moreover, RBF 130 can be reconfigured so that a different channel group is blocked. In which case, demultiplexers must be added to a different port or slot of router 180, for example. Since add/drop multiplexers are often deployed in remote locations, service personnel must travel to the add/drop multiplexer site and physically attach the channel group demultiplexer to a new output port of router 180.
Alternatively, RBF 130 can be replaced with a so-called wavelength selective switch or WSS 210, as shown in FIG. 2. Wavelength selective switches are known components that coupled to multiple input and output lines, and can selectively block optical signals on a per wavelength basis. In this instance, WSS 210 is coupled to input lines 209, 213 and 215, and output lines 222, 225 and 226. The operation of routers and group demultiplexers is similar as that discussed above in regard to FIG. 1. However, as further shown in FIG. 2, additional routers can be provided, each one coupled to a corresponding one of the input or output lines. However, the WSS-based add/drop multiplexer shown in FIG. 2 suffers from similar disadvantages discussed above with respect to FIG. 1. Namely, any reconfiguration of WSS 210 resulting in a change in the wavelengths to be added and dropped requires physically coupling channel group demultiplexers to a different router output port.